For the dynamic correction of viewing defects it is known to use spectacles having spectacle lenses which may be adjusted electronically in their refractive power. In U.S. patent application publication 2006/0012747 A1 there is described an auto-focusing spectacle in which a sensor is located on the spectacle frame which measures the refractive power of at least one eye directly. The output signal from the sensor is transmitted to a control unit which adjusts the refractive power of the spectacle lens associated to the measured eye accordingly. By doing so, also persons having a limited ability of accommodating are enabled to view sharply over a wide range of distances.
A similar apparatus is also described in U.S. Pat. No. 6,491,394 B1. The electronically controlled spectacle disclosed therein also dynamically corrects viewing defects of the spectacle user. These defects comprise defects of lower order (short sightedness, long sightedness, astigmatism), but also viewing defects of higher order (aberrations and further viewing defects of higher order). In these prior art spectacles one has, further, provided for adjusting the refractive power in limited areas thereof.
U.S. Pat. No. 4,300,818 discloses an auto-focusing spectacle. On the rear side of the spectacle frame nose piece there are provided two light sources and two light detectors in a position and orientation being specific for the particular user of the spectacle. Due to their particular orientation, the light sources each emit a light beam directed onto the bright sclera of the right eye and on the dark cornea of the left eye, respectively, when the user looks at a far away object. The light detectors receive the reflected beams. If the user now looks at a nearby object, the so called convergence angle appears, i.e. the axes of the eyes which extended in parallel for a far away object now intersect for a nearby object with the convergence angle increasing as the object distance decreases. As a result, the emitted light beams no longer impinge on the illuminated bright and dark areas within the eyes but on areas of opposite reflectivity. Accordingly, the convergence angle may be determined.
For viewing defects of higher order, various influences are of importance. For example, one has found out that the illumination intensity, i.e. the brightness of the light impinging into the optical observation instrument and, hence, on the eyes of the user, plays a significant role. This is because the pupils of the user adapt as a function of the illumination intensity, in that the pupils are smaller under bright ambient conditions and are wider within darkness, as is generally known. This variation in pupil diameter substantially affects the viewing defects of higher order.
On the one hand, the viewing defects of second order (defocus and astigmatism) vary together with the pupil size. This is because the cornea does not have a spherical surface but only an essential toric surface. Therefore, for different pupil diameters, the cornea surface used is different. For irregular cornea surfaces, this results in a variation of the effective main cornea curvature. This alters the total refractive power of the eye. For some people, therefore, the refractive power of their eyes is higher at night.
On the other hand the viewing defects of higher order vary with pupil size. Even if the cornea had an ideal spherical surface, a variation of pupil diameter would cause a variation of the spherical aberration of the eye. Further, also the irregular cornea surface plays a role also in this context. For different pupil diameters the effect of irregularities is different. This results a change of higher order aberrations.
It is, therefore, an object underlying the invention to improve an optical observation instrument, in particular a spectacle, as well as method of the type specified at the outset, such that the afore-mentioned disadvantages are avoided. In particular, the invention shall make it possible to perfect the correction of viewing defects of second and higher order by further taking into account additional influences.
In an optical observation instrument of the type specified at the outset, this object is achieved according to the invention in that the sensor is a brightness sensor.